Application of chopper devices to electric cars is being employed generally. Recently, instead of this application, a system of driving an electric car by using a VVVF inverter and an induction motor has been proposed. Utilization of a chopper and DC motor system having the specific feature of the proposed system also has been desired because of its high reliability. Thus, the conventional chopper is considerably effective in that the maintenance is simple and the resistance loss is small because the contacts thereof have been eliminated, and in that the electric power is economically used because of the effect of regenerative braking; however, the conventional chopper involves problems needing to be solved.
Heretofore, a chopper control device for an electric car generally employs a system in which a field chopper and a DC compound motor are used, or a system in which a chopper and a DC series motor are employed. However, the former is disadvantageous in that, because of the compound motor, during power running operating and during regenerative braking operation the motor has different characteristics because of the cumulative and differential connections with the series field and shunt field. Furthermore, a cam type controller is required because the armature circuit is of the resistance control type. On the other hand, the latter is disadvantageous in that switching means are required for switching between the power running operation and the regenerative braking operation, and for switching forward movement and backward movement, and furthermore the braking operation is unstable during high speed operation because the motor voltage must be lower than the line voltage (or supply voltage) when the regenerative braking operation is effected.